SIBO explained: symptoms, causes, testing and treatment

November 7, 2026

Bloating that won't settle, feeling full after a few mouthfuls, gas, loose stools. These are common complaints, and small intestinal bacterial overgrowth, usually shortened to SIBO, is one of the explanations a doctor may look into. It is also a term that gets used loosely online, so it helps to know what it actually means, how it's tested for and where the evidence on treatment is solid or thin.

What SIBO is

SIBO means there are more bacteria in the small intestine than there should be, often including types not usually found there, according to the StatPearls reference chapter on SIBO. An older name for it is blind loop syndrome.

The small intestine, where most nutrients are absorbed, normally carries relatively few bacteria compared with the colon. Several defenses keep it that way. Stomach acid kills many microbes before they get far. Bile and antibodies in the gut lining help keep numbers down. And between meals, waves of muscle contraction called the migrating motor complex sweep contents, bacteria included, further along the gut.

When one or more of those defenses fails, bacteria can build up. If fluid from the small intestine is sampled and cultured, the North American consensus on breath testing suggests defining SIBO as more than 1,000 colony-forming units of bacteria per milliliter.

Symptoms

SIBO's symptoms overlap heavily with IBS and other gut conditions, which is part of why it can be hard to pin down. The Mayo Clinic and StatPearls list these among the most common:

bloating and a swollen-feeling abdomen
abdominal pain or discomfort
diarrhea
excess gas
nausea
an uncomfortable feeling of fullness after eating
loss of appetite

When the overgrowth is more severe or goes on for a long time, it can interfere with absorbing nutrients. That can lead to weight loss you didn't intend and to vitamin deficiencies, particularly vitamin B12, which can in turn cause tiredness, weakness and nerve-related symptoms.

Who is more likely to get it

SIBO usually develops when something weakens the gut's normal defenses. Slow movement through the gut is one of the main routes. Conditions that impair the migrating motor complex, including diabetes, an underactive thyroid and scleroderma, raise the risk. Opioid painkillers slow the gut as well, and a 2024 review of SIBO in Current Infectious Disease Reports recommends cutting back on them where possible as part of treatment.

Changes in the gut's structure are another route. Scar tissue (adhesions) from earlier abdominal surgery, small pouches in the wall of the small intestine called diverticula, narrowed sections known as strictures and some operations, including gastric bypass, can all leave places where contents linger and bacteria accumulate.

Low stomach acid, whether from a medical condition or from acid-reducing drugs called proton pump inhibitors, removes one of the first barriers, and immune deficiencies can raise susceptibility too. SIBO is also more common alongside several other conditions, including Crohn's disease, celiac disease, chronic pancreatitis, irritable bowel syndrome and bowel damage from radiation treatment.

SIBO and inflammatory bowel disease

The overlap with inflammatory bowel disease (IBD) has been studied in some detail. A 2025 meta-analysis in Frontiers in Medicine pooled 29 studies that included 3,250 people with Crohn's disease or ulcerative colitis, and about 31% of them tested positive for SIBO. In the 11 studies that also tested healthy people, those with IBD had roughly five times the odds of a positive result (an odds ratio of 5.25). SIBO was more likely in people with a lower body mass index, more bloating or flatulence, previous abdominal surgery, or Crohn's disease that had caused strictures or penetrating complications.

The authors are clear about the limits. None of the studies used culture of small-intestine fluid, results varied a great deal between studies, and the type of test made a big difference: lactulose breath tests found SIBO in about 43% of people with IBD, glucose breath tests in about 22%. The authors suspect the pooled figure overestimates the true rate.

How SIBO is diagnosed

No test for SIBO is perfect. The reference standard is to take a sample of fluid from the small intestine during an upper endoscopy and culture it. That is invasive and not widely available, and the 2024 review notes that the sample can pick up microbes from the mouth and throat on the way down. The consensus panel itself judged current culture techniques unsatisfactory.

Breath tests

Breath testing is the less invasive alternative. You drink a sugar solution, either 75 grams of glucose or 10 grams of lactulose, then give breath samples at set intervals. Bacteria that ferment the sugar release hydrogen, which is absorbed into the blood and breathed out. The test also measures methane, which comes from a different kind of organism.

Preparation affects the result, so the consensus sets out ground rules. Your clinic's instructions may differ in the details, and you shouldn't stop any prescribed medicine without checking with the person who prescribed it. The main points are:

no antibiotics for four weeks before the test
stop laxatives and drugs that speed up the gut at least a week before, if you can manage without them
avoid fermentable foods such as complex carbohydrates the day before
fast for 8 to 12 hours beforehand
don't smoke on the day of the test, and keep physical activity to a minimum while it runs

Under the consensus criteria, a rise in hydrogen of at least 20 parts per million (ppm) above the starting value by 90 minutes suggests SIBO.

Methane is read separately, and the consensus counts a level of at least 10 ppm as positive. The methane is made by archaea, single-celled organisms that are not bacteria, so this result is now called intestinal methanogen overgrowth (IMO) rather than SIBO. You'll still see the older label, methane-dominant SIBO, in some places. The consensus links excess methane with constipation and slower movement through the gut, and a test can show both a hydrogen rise and raised methane at the same time. IMO has its own explainer on this site.

Breath tests have real limitations. The 2024 review reports that their sensitivity and specificity vary widely between studies, that up to 20% of healthy people can test positive, and that people with severe problems of gut movement can test falsely negative. The IBD figures above show how much the choice of sugar can change the answer. A result has to be read alongside your symptoms and history.

Treatment

Antibiotics

Antibiotics are the main treatment, and rifaximin is usually the first choice. In the 2024 review, a 14-day course resolved symptoms in about half of patients. When rifaximin doesn't work or isn't suitable, doctors may use other antibiotics, such as amoxicillin-clavulanate, ciprofloxacin, doxycycline, metronidazole or trimethoprim-sulfamethoxazole, although the review notes there is little high-quality evidence comparing them with rifaximin.

Treating the underlying cause

Antibiotics reduce the bacteria but don't fix whatever allowed them to build up. Longer-term management means looking at that cause, which might mean better control of diabetes or thyroid disease, treating IBD, reviewing medicines that slow the gut or lower stomach acid, or occasionally surgery to correct a structural problem.

Diet

Elemental diets, in which a liquid formula of pre-digested nutrients replaces all food for two to three weeks, have been shown in earlier studies to improve symptoms and bring breath tests back to normal. They are hard to follow, and the evidence is still limited. We look at the recent studies, including how small some of them were, in our article on what newer gut microbe research means for IBS.

What to eat after treatment is less clear than many online protocols suggest. The 2024 review says diets to keep SIBO in remission haven't been rigorously studied. Its authors suggest a diet low in fermentable foods, with at least five hours between meals, but that advice is based on theory and on data from IBS rather than on trials in people with SIBO.

Why SIBO often comes back

Relapse is common. The same review reports that SIBO returns in up to 44% of patients within nine months of successful treatment, and some people need more than one course of antibiotics. Drugs that stimulate gut movement, called prokinetics, have some evidence for delaying relapse, but it comes from small, older studies. This is one reason the underlying cause gets so much attention once the first round of treatment is done.

When to see a doctor

The Mayo Clinic advises getting checked if diarrhea or abdominal pain lasts more than a few days, and getting care right away for severe abdominal pain. Weight loss you didn't intend is also worth raising with a doctor. Because SIBO overlaps so much with IBS, celiac disease and IBD, a proper assessment is worth more than a home test or a protocol found online. If you already have an IBS diagnosis, our guide to IBS basics explains how that diagnosis is made.

This article is for general education and is not medical advice, so please talk to a doctor or other qualified clinician about your own symptoms, testing and treatment.